Mariner 10 in 1974 -- only spacecraft to visit

Very moonlike, old surface, many craters

Mercury has a large iron core 3/4 diameter

Exciting news-- may be ice at the poles, in the shadows of craters that never see light. Ice would have stayed after a comet impact. The same thing may even be true of our Moon

No atmosphere

After Mercury had collected craters, caused by leftover planetesimals, it apparently shrank 2-4 km in diameter. Stresses on its crust--made huge faults where one part of the crust tried to ride up onto another, almost like plate tectonics. Due to cooling of planet and formation of core

Much of the solar system was explored in the 60’s, 70’s and 80’s (Pioneer, Voyager, Viking).

Venus remained an enigma because of its global sulfuric acid cloud cover

Exception: Soviet exploration included landers on Venus that imaged the surface

Cobbled together from old spare Voyager parts, the spacecraft that had been sent out earlier to Jupiter, Saturn, Uranus and Neptune

Launched 1989

Had only one instrument, an imaging radar system

Radar has wavelength of 4 cm, and can see through the cloud

Mapped the whole planet, things as small as 200 m

Venus: Earth’s twin--almost same size, composition, gravity, and distance from Sun

Venus spins backwards--maybe due to a giant impact flipping it over

Day is 117 days, but this is its synodic period --noon to noon

Sidereal day - one rotation with respect to stars, is 243 days

Its year is 225 days. Year is less than its day

Very, very hot, heavy atmosphere, covered in clouds

CO2, N2 atmosphere

Venus is way too hot to have liquid or solid water -- but there’s a tiny bit in the atmosphere. So it doesn’t have oceans

But it has large, continent like features, and low lying volcanic plains

Recall that there is a compositional difference between oceanic and continental crust

Is the same true for Venus--are the highlands lighter rocks floating on heavier rocks?

Is there plate tectonics on Venus?

Sends out radio pulses

They bounce off rocks, some rocks reflect more than others

Pulses are received, computer reconstruction of surface

Radar is not light -- bright means rough or electrically conductive

Dark means smooth or an insulator

Here’s an image of the LA basin, using radar. Can see right through clouds, but things don’t look quite right. Mountains

Have to keep this in mind when we’re looking at Magellan Radar images

Bright and rough continental area, Alpha Regio. Looks like scrunched up crust

Dark, patches around craters that may be related to dust kicked up from the impacts

Highest mountains -- 12 km high (40,000 ft). Very reflective, could be solidified metal, spewed out from volcanoes

Going E around Venus, we see the beginnings of a huge continent-like area, called Aphrodite. Shot through with enormous chasms that look like giant rift zones. At first we thought that it might have been plate tectonics, but there is no evidence anywhere for subduction zones

Large circular regions may be partial subduction zones, or they may be places where huge plumes came up from the mantle, and spread out under the lithosphere. They are called coronae

In addition to 10’s of enormous volcanoes, (600 km across, 5X bigger than Moana Loa, biggest volcano on Earth), may 10,000s of small shield volcanoes, 20 km across.

There are about 910 craters on Venus, none smaller than 1.5 km. This is because the atmosphere is so thick that any impactor smaller than 150 m across gets burned up. These craters formed on the volcanic plains. Notice the bright impact ejecta--rough, blocky material thrown out of the craters

Largest impact crater on Venus, 280 km across.

Named after Margaret Mead--all feature on Venus named after women

Except Maxwell, after James Clerk Maxwell, studied light and radio

There are lots of regions of scrunched up crust (more violent than on the Earth), called tessera. There are very few impact craters on the tessera, indicating that the surface has been so recently that the craters have been wiped out. We call this a young surface, like the Earth

So we have only 910 craters, with maybe 34 of them modified. Venus has wiped out all the older craters, but there is no real evidence of how they got wiped out--the modified ones aren’t enough

This has yielded a theory called catastrophic resurfacing -- 500 my ago, the entire lithosphere of Venus became unstable, and reworked itself in 100 my. It turned over in one enormous burp, followed by the building up of volcanoes, and the wholesale cracking and buckling of the crust. But no plate tectonics -- that is a continual process.

Upwelling of thick, viscous lava, forming these pie shaped bubbles of magma under the surface

Even stranger volcanic features, small volcanoes called arachnoids, because they look like spiders

You can even see a vent, and the intense tectonic fractures around it

Huge circular features, but ranging in size from 100 km to 2600 km. May be the surface expression of huge plumes from the interior, or maybe where crust is subducting slowly

Still, no global pattern of plates like we clearly see on the Earth

Magellan took three kinds of data -- images, altitudes, and gravity.

We have been looking at the images

Gravity tells us the density of stuff beneath the crust, so that we can tell if some things are continents, or if volcanoes have deep roots

Altitude tells us how high things are

If we use a computer to combine the image and altitude data, we can simulate any perspective, even ones the spacecraft never had.

This is a volcano, where you can see recent bright flows that came off the summit. The dark, smooth area is old lava flows, maybe lava plains that existed before the volcano

The same is true of craters split by fractures--there are very few

Here we see one of the many, many cracks, but only one disrupted crater.

Even though Venus has a very thick atmosphere, there are only weak winds at the surface. The reason is that the air conducts the heat very efficiently, and temperature differences from place to place, that drive winds, don’t build up.

But there is some wind, and even some sand dunes. But we know it isn’t too common, because we don’t see sand all over the place, like in craters

The Soviets were much more aggressive in sending missions to Venus

They sent a number of landers, which sent pictures for one hour (1970s)

Here’s what the surface looked like

Rocks, soil like surface, a little bit of sky

Image is distorted because of the camera optics.

Looks like a volcanic plain

Another mystery of Venus is a network of channels, some 6000 km, and going uphill! They couldn’t have formed from water, because it’s too hot.

We know that it’s been hot since the catastrophic event, because there are no small craters, the air has been thick, and the greenhouse effect has prevailed

Channels might be some kind of lava tube, but there are no lava tubes on Earth anywhere near this long--long as the longest rivers on Earth

At the end of the long channels, or canali, there are river delta features

It looks like the liquid that made the channels ended its journey here, but they look a lot like terrestrial river deltas

A scientist at USGS has come up with a suggestion to explain these river delta features

They could be formed by low temperature lavas, ones that stayed liquid for a long time on the hot Venus surface, and flowed like water

He even proposed that this stuff could still be liquid, in under ground rivers of liquid carbonate

Highly organized cracks that extend for thousands of km. The whole surface of Venus experienced huge stresses, apparently in a globally synchronous event.

One theory (mine) is that climate change, induced by volcanic release of greenhouse gases, heated and cooled the surface enough to crack it.

Half the size of Earth

24 hour day, same as Earth

23 deg axial tilt, same as Earth --which means it has seasons just like Earth

Half as much sunlight as Earth

Very cold, -50 F on average

But in the summer on the equator, it can get room temperature

Has a thin CO2 atmosphere, 1/100th of the Earth’s.

Almost same composition as Venus’, but 10,000 times thinner

Has polar caps, made of water ice and dry ice (CO2)

It has weather, with morning clouds, snow, storms, dust storms

But no evidence for running water recently (recall surface dating with crater counting). It’s all frozen out.

Here’s what Viking saw

Mars has two kinds of terrain, roughly divided between the Northern hemisphere and Southern hemisphere

The northern hemisphere, which we are looking at here is smooth, relatively crater free

It has huge shield volcanoes, sitting on a huge bump on the surface, as well as other places.

It also has an enormous crack, the size of the US, splitting down the side of the bump

The northern hemisphere has relatively few craters, probably wiped out by the volcanism--it is a relatively young surface, maybe 1 by old

Notice the clouds on the morning terminator--morning fog surrounding three enormous volcanoes on the Tharsis bulge

Light is where windblown dust has settled -- dark areas may be bedrock

Mars is red because it the surface has oxidized iron -- rust, just like volcanic red dirt on the Earth

The southern hemisphere is much, much older, as evidenced by a lot of craters

The entire south is higher than the volcanic plains, and we call it the highlands, just like on the moon

Some people have even suggested that Mars may have had oceans, and that there are ancient shorelines

This is the scarp that makes a boundary between the south and north

you can see a crater with lobate ejecta in the center right

Channels ending between s and n

Sand dune fields

Some craters have funny impact ejecta, called lobate ejecta

It looks like the impactor hit where there was subsurface ice

The ice melted on impact, made mud, which was flung out in these patterns

In fact everywhere we look, in both the old (South) and new (north) areas of Mars, we see evidence for running water in the past

Here is a very, very old valley network

The valleys are more common in the old regions, but there is also evidence for a large system of drainage networks from older region to newer regions

Some of the river systems clearly look like rivers, and must have carried enormous amounts of water

We interpret this to mean that Mars was warmer in the past

It has almost no greenhouse effect now, because the air is so thin

But it may have had a much thicker CO2 atmosphere in the past, now sequestered at the poles or escaped from the planet

Mars has a lower gravity, and light gases can escape easily

Higher pressure, more greenhouse, liquid water

Now the water is probably frozen as permafrost, or under the ground

There is other evidence for massive climate change as well

But there is other evidence for flowing water

Here, you see a teardrop shape around a crater. Somehow when the crater formed (before the flood) it sort of cemented the ground, making it more resistant to erosion as the water flowed

There are places where it looks like huge amounts of ice melted suddenly and flowed out. The land collapsed in these places, creating chaotic terrain

There are places like this on the Earth -- badlands of Eastern Washington

Most of the intricate river networks are ancient -- 3.5 B years ago

Recall that the solar system is 4.5 billion years old

But there is more recent evidence of flowing water

This is the result of a series of great floods

You can tell that the water has moved fast, cutting deep channels

These may have been much more recent -- maybe 1 By ago

The southern portion looks just like the moon, like a dead planet

This is a huge basin and numerous other craters in the south

When we sent Mariner 9 there in 1971, it flew under the south pole, and we thought Mars was just like the Moon. It was very disappointing

But Viking changed all that

Valles Marineris definitely was not formed by water, because we don’t see any evidence of sediments

But we do see a lot of carved away cliffs, shown in this oblique image of Candor Chasma

When we look very closely at the cliffs, we see layers at the top, just like layers on the Earth

These may have been put down by volcanic events, or may be rocks that reacted with gases in the atmosphere when it was different (volcanically active)

The polar caps grow and shrink with the seasons

But there is a permanent core, that looks like it may have huge ice walls and canyons

This image was taken in the late summer, when the pole is lit up and about to plunge into a midnight sun, when the cap will grow

As the cap sublimates (grows or shrinks), a huge polar vortex of air is generated, circulating about the cap.

you can see how the cap get laid down by the intricate spiral pattern

Because the caps might be sources of water, and give a clue to Mars’ climate, Viking looked very hard there

This is near the edge of the N polar cap, where there are probably large cliffs and canyons of ice

There are intricate lines of alternating ice and dirt.

This is evidence for climate change on Mars

There were times when ice was deposited and taken away, and times when dust or dirt was deposited or taken away

The caps have experienced a varying climate

The biggest volcano in the solar system

80,000 feet high, 500 miles across

Called Olympus Mons

Enormous pit in the center

Calderas are formed when the volcano finished erupting, and the magma from below withdraws. The ground collapses at the top

Because it is so high, Olympus Mons strongly influences the weather on Mars, just like mountains do here

Here is an oblique view of the summit, encircled by clouds

Dunes composed of dark sand grains encircle the north polar cap of Mars. This view was taken during the northern summer in May 1999.

Since we didn’t know what to expect, we surveyed possible landing sites for several months before deploying the landers

The lander used parachute and rockets, and used the orbiter as a relay

Landers were semiautonomous, and bristling with instruments

One thing about the wind on Mars -- it can get very strong

It can get so strong that dust storms kick up during the warmest weather in the south

Most dust storms are local, but they can all combine to cover the planet completely

This is a series of images (taken at the same time of day) showing how everything got darker during a dust storm

Rock seen at the V1 landing site

About two meters across, called ‘Big Joe’

You can see where sand from dust storms have settled on it

But you can also see where the sand slumped off

This happened while the lander was there -- we saw it before and after

Viking scooped up soil with a shovel, and put it in various chambers for testing

They tested what elements the rocks were made of -- Fe, Mg, Al --basalts

They looked for water (1%)

I know a researcher who discovered you can retrieve the water from this kind of soil with a microwave oven

You can imagine the anticipation we felt, wondering what this new place, new world looked like.

Far more Earth like, this is even a place that could be home to humanity

Here’s what it saw

Rock strewn desert, light sky

The rocks are probably strewn from some distant impact crater or volcano

If you look at the rock on the lower right, it looks vesicular

That means it has evidence of bubbles that came out as the rock cooled

Probably volcanic in origin